1. Field of the Invention
The invention is a Permanent Magnet (PM) Flux-Biased Magnetic Actuator with Flux Feedback for providing magnetic suspension and control forces having a linear transfer characteristic between force command and force output over a large gap range. Applications for the invention may include magnetic actuators for shape and figure control of antennas and precision segmented reflectors, magnetic suspension for momentum/energy storage devices, and payload pointing, isolation, and control systems.
2. Description of the Prior Art
Typically, the force produced by a conventional single magnetic suspension actuator element is an attractive force that is directly proportional to the square of the flux in the magnetic gap. The flux, under ideal assumptions, is directly proportional to the current in the actuator element coil and inversely proportional to the magnetic gap. In order to produce a bi-directional force, prior art devices utilize two or more actuator element pairs acting together along a single axis.
Many different approaches for controlling and linearizing the force produced by this type of magnetic actuator have been investigated as described in U.S. Pat. Nos. 4,629,262 and 4,642,501 to Brain J. Hamiliton and Kevin D. Kral, respectively, and NASA Publication No. CP-2346, dated Nov., 1984, entitled "Overview of Magnetic Bearing Control and Linearization Approaches for Annular Magnetically Suspended Devices" by the inventor herein, Nelson J. Groom.
In conventional applications, current feedback has typically been used to control the current in the actuator coils and thus indirectly the flux in the magnetic actuator gaps. In terms of the simplicity and efficiency of known magnetic actuators, the use of a permanent magnet PM to provide flux bias approach has been found to have certain advantages because: (1) a linear relationship between force and current at a given operating point can be obtained by simply controlling the actuator elements differentially about the bias flux established by the permanent magnet, PM; and (2) since the bias flux is supplied by permanent magnets PM, the continuous power dissipation that results when bias currents are used is eliminated.
The major disadvantage of known PM flux biased magnetic actuators such as described in the aforementioned NASA publication, No. CP-2346, is that the equivalent linear actuator gain, which is exhibited at a given operating point, changes when the operating point changes. This has prevented PM flux biasing from being used in applications which require constant actuator gains over a large gap range. Also, from a control system standpoint, the conventional PM flux biased actuator has a minimum bandwidth requirement which can limit or prevent its use.
As is known, a continuous, variable bias current approach requires complicated electronic circuitry and the use of analog function modules such as multipliers which in turn can introduce accuracy problems. The aforementioned U.S. Pat. No. 4,629,262 to Hamilton discloses an improvement on the variable bias current approach which uses a force sensor in a feedback loop to provide both displacement information and force linearization.
However, in addition to the disadvantages inherent in the variable bias current approach, the use of a force sensor as so described requires mounting the force sensors on the suspended element. This means that the output signal from the sensors must be transfered across the gap by cable or some type of complex, noncontacting signal transmission technique. Also, the commercially available force sensors that meet the necessary accuracy and bandwidth requirements are fragile, expensive, and require complicated support electronics.
An improvement on both the force feedback approach and variable bias current approach is disclosed in U.S. Pat. No. 4,642,501 to Kral et al in which flux feedback is used to adjust the continuous bias currents while avoiding the use of a force sensor and necessary complicated gap/current computations. The use of continuous bias currents still results in a continuous power dissipation in each of the actuator elements. This as mentioned can significantly reduce the efficiency of the system and lead to heat related problems.
For example, to minimize peak power the bias current will in general be half the maximum current driven through a given coil. This means that for maximum force in a given direction, the current in one coil will increase from a value equal to the bias current to a value equal to maximum and the current in the opposite coil will go to zero. Since the maximum power dissipation is equal to the maximum current squared time the resistance of the coil, the nominal continuous power dissipated in one coil due to the bias current will be one-fourth the maximum (if the suspended element is centered and the force command is zero). For one pair of opposing elements then the total quiescent power dissipation would be one-half the value required to generate maximum force in a given direction.